After moving a Joomla 2.5 site to a new URL (i.e. new domain name) the backend (administrator) did no longer let me login as admin (or any other account for that matter). I tried to recover the admin account by setting the password field to a known value directly into the MySQL database described here, but it didn’t help.
The reason was that the cookie domain was set in the global configuration and this domain was different from the new domain where the site was installed. By editing the configuration.php file and emptying the $cookie_domain parameter I was able to login again, i.e:
public $cookie_domain = '';